1. Field of Invention
The present invention relates to a Zn-based composite-plated metallic material exhibiting improved, corrosion-resistance and other properties and to a composite plating method. The present invention relates to mainly steel materials plated with Zn or Zn alloy. Specifically, the present invention relates to a Zn-electroplated steel sheet exhibiting improved, corrosion-resistance, paint-adherence, formability, and other properties, as well as a method for producing said steel sheet.
2. Description of Related Arts
The Zn-electroplating is mainly carried out as the rust-proofing plating of steel sheets and has been broadly used in the field of automobiles, household appliances, and the like, by utilizing the sacrificing (galvanizing) anode effect of the Zn coating on the Zn-plated steel sheet. Advantageously, the Zn electroplating is superior to the other plating methods, such as hot-dip galvanizing, in uniformity, formability, smoothness, and the like of coating and enables thin deposition. Nevertheless, along with demands for further improving the corrosion resistance and the other properties centered recently on automobile bodies have been enhanced, it became more important to develop Zn-based electrolytic plating having further higher properties than the pure Zn plating.
The technical developments, which have been made in the field of plating so as to meet the above demands and to improve such surface properties as the corrosion-resistance and paint-adherence of Zn-electroplated steel sheets are roughly classified into two methods. One of them is already broadly employed for a composite electroplating, and resides in alloy-plating by means of a plating bath, in which such metallic ions as Sn, Pb, Ni, Mo, Mn, Fe, Cu, Co, Cd, and the like capable of codepositing with Zn are incorporated. The other is a dispersion-plating method with the use of additives of organic resin or inorganic material. As the inorganic materials used, fine particles of compounds insoluble to the bath, such as alumina, silica, and the like are suspended and dispersed in the bath.
Since, in the ordinary Zn-plated steel sheets, the potential difference between the constituent Zn of plating layer and the Fe as the substrate material is great, the sacrificing anode-effect of the plating layer is excellent. This fact, however, makes it necessary to form an extremely thick plating layer for the purpose of rust-proofing over a long period of time, because the dissolution rate of Zn-plating layer is rapid. The Zn-alloy plating method of Ni, Co, Fe, and the like is presently used to cope with the above described problems, and is based on the concept that the potential difference between the substrate material and plating layer is appropriately controlled by means of depositing, together with Zn, a metal which is electrochemically more noble than Zn, thereby adjusting the sacrificing anode-current (galvanic current) within an appropriate range and hence controlling the corrosion rate of plating layer as low as possible.
The presently used, Zn-alloy plating method intends therefore to attain mainly the electrochemical, sacrificing corrosion-proofing. Therefore, the Zn-based alloy plating allegedly exhibits a corrosion-resistance over a longer period of time compared with the ordinary Zn-plated steel sheet, using the identical deposition amounts. Nevertheless, the Zn-based alloy plating involves a limitation in improvement of the corrosion resistance, since a too high content of noble metal incurs a decrease in the sacrificing anode effect, pitting corrosion is liable to occur. In addition, a uniform dissolution of the respective elements of the plating layer is a premise for realizing the excellent corrosion resistance of the Zn-based alloy. Actually, however, a preferential dissolution of Zn, which is potentially less noble, occurs, with the result that the proportion of noble metal(s) to less noble metal(s) of the plating layer increases, and finally the potential of the plating layer arrives that of noble metal(s). In this case, the galvanic corrosion-proofing of the iron of substrate is lost. This is presumably a reason for the pitting corrosion described above.
In the dispersion plating method, the fine particles of silica, titanium oxide, and the like are dispersed in the liquid body and are incorporated in the plating layer, thereby lessening the electric conductivity and hence suppressing the corrosion speed of a plating layer to the level as low as possible. This technique involves a difficulty in effectively incorporating the fine particles in the plating layer. In addition, since the fine particles used must be chemically inactive so as to prevent dissolution thereof in the plating liquid, the effects of composite particles are principally attributable to physical protection, alone. This provides a limitation in improvement of the corrosion resistance.
Incidentally, Al is not capable of co-depositing with Zn (c.f. Iron and Steel Handbook, 3rd Edition, Volume VI, page 419, FIGS. 10,27). However, according to a known technique for providing a Zn-Al composite, electroplated steel sheet, the form of Al in the plating bath is modified to a special one so as to co-deposit the same together with Zn. The metallic Al particles, which are dispersed in a plating layer, have a property of being liable to dissolve during the corrosion. This property is utilized to chemically and electrochemically enhance rust proofing. More specifically, in Japanese Examined Patent Publication No. 54-30649, a technique for forming a Zn-Al composite electroplating containing 1.5 to 70% by weight of the dispersed Al, by means of stirring the Zn plating bath, in which the Al powder is suspended is disclosed. The technique described in this publication has common characteristics of alloy plating and dispersion plating. The structure of obtained plating is however so prominent that Al is enriched and segregated in the top surface side of a Zn-Al composite electroplated layer, and such Al allegedly improves the corrosion-resistance according to the description of publication.
In addition, according to Japanese Unexamined Patent Publication No. 60-125,395, there is a technique disclosed for forming a Zn-alumina series composite plating containing, in terms of Al.sub.2 O.sub.3, from 0.01 to 3% by weight of aluminum compound, which is soluble to dilute hydrochloric acid, by use of the plating bath, in which positively charged alumina sol is added.
As is described above, the alloy plating method involves a tendency that the galvanic corrosion-protection by Zn is weakened by the alloyed noble metal. The mere alloying of a plating layer therefore involves a limitation in the improvement of corrosion resistance.
In the dispersion plating of fine particles, the fine particles 5 m.mu. to 50 m.mu. in diameter are positively charged by means of a cationic surfactant agent and move toward the surface of a cathode due to electrophoresis, and deposit on the electrode surface while losing the charges. The fine particles may have positive charges due to the inherent characteristics thereof. The deposits on the electrode surface are merely physically adsorbed due to the Van der Waals force with respect to the electrode surface. Contrary to this, the bonding between the deposited plating metal and the constituent metal of an electrode is a metallic bond. Accordingly, the fine particles can easily separate from the electrode surface, until such a deposition state of matrix metal that the fine particles are embedded therein at a half or more of the diameter of fine particles. The drawbacks of the conventional dispersion plating method are therefore as follows.
(1) The co-depositing plating is not obtained at a plating thickness of 1/2 or less relative to the diameter of co-deposited particles.
(2) Since the separation possibility of co-depositing particles from the plated surface is high during the plating process, a high co-deposition degree is not obtained. In order to obtain a certain co-deposition degree, the content of co-depositing components in the bath must be raised exceedingly.
(3) Under the conditions of a high speed-plating (plating at a high current density), vigorous stirring becomes necessary. This incurs a decrease of the co-deposition degree.
(4) With the increase in the particle diameter, the influence of gravity appears, such that, for example in the plating using horizontal electrodes, the co-deposition degree in the lower side is lower than that in the upper side of a workpiece.
(5) The solid matter included in the bath makes it difficult to maintain the uniformity of bath components and to maintain the devices.